The Kulaks were slightly richer 'forms' of peasants, who owned their own farms, animals and land. They made money for themselves and were viewed by communist propaganda as 'selfish' and 'capitalist' for doing this.
Due to the fact they had worked very hard for their farm, and were making somewhat of a sustainable living from them, they were understandably the biggest group to oppose the idea of collectivising.
The poorer peasants disliked the Kulak class, as they often exploited such poorer peasants as workers for them, and took all the good farm land.
They were believed to be a dangerous class for the economy.
This opposition towards the kulak class escalated when Stalin ordered to "Liquidate the Kulak Class".
At first, they were forbidden from joining new collective farms, but then he ordered for them to be removed from society entirely.
Some were rounded up by dekulakisation squads and shot on the spot.
Two million of the kulaks were loaded into cattle trucks and imprisoned in Serbian labour camps.
Others were given a small patch of bad land as their 'farm', but even these Kulaks ended up dying often of starvation due to the impossibility of farming the land they were allocated.